Correlations of two flow harmonics $v_n$ and $v_m$ via three- and four-particle cumulants are measured in 13 TeV $pp$, 5.02 TeV $p$+Pb, and 2.76 TeV peripheral Pb+Pb collisions with the ATLAS detector at the LHC. The goal is to understand the multi-particle nature of the long-range collective phenomenon in these collision systems. The large non-flow background from dijet production present in the standard cumulant method is suppressed using a method of subevent cumulants involving two, three and four subevents separated in pseudorapidity. The results show a negative correlation between $v_2$ and $v_3$ and a positive correlation between $v_2$ and $v_4$ for all collision systems and over the full multiplicity range. However, the magnitudes of the correlations are found to depend strongly on the event multiplicity, the choice of transverse momentum range and collision system. The relative correlation strength, obtained by normalisation of the cumulants with the $\langle v_n^2\rangle$ from a two-particle correlation analysis, is similar in the three collision systems and depends weakly on the event multiplicity and transverse momentum. These results based on the subevent methods provide strong evidence of a similar long-range multi-particle collectivity in $pp$, $p$+Pb and peripheral Pb+Pb collisions.
The symmetric cumulant $sc_{2,3}\{4\}$ results as a function of multiplicity ($N_{ch}$) in pp collisions at $\sqrt{s_{NN}}$ = 13 TeV
The symmetric cumulant $sc_{2\,3}\{4\}$ results as a function of multiplicity ($N_{ch}$) in pp collisions at $\sqrt{s_{NN}}$ = 13 TeV
The symmetric cumulant $sc_{2\,3}\{4\}$ results as a function of multiplicity ($N_{ch}$) in pPb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV
A combination of searches for resonant Higgs boson pair production is presented, using up to 139 fb$^{-1}$ of $pp$ collision data at $\sqrt{s}=13$ TeV recorded with the ATLAS detector at the LHC. The combination includes searches performed in three decay channels: $b\bar{b}b\bar{b}$, $bb\tau^+\tau^-$ and $bb\gamma\gamma$. No excess above the expected background is observed and upper limits are set at the 95% confidence level on the production cross section of Higgs boson pairs originating from the decay of a narrow scalar resonance with mass in the range 251 GeV-5 TeV. The observed (expected) limits are in the range 0.96-600 fb (1.2-390 fb). The limits are interpreted in the Type-I Two-Higgs-Doublet Model and the Minimimal Supersymmetric Standard Model, and constrain parameter space not previously excluded by other searches.
Local p-value as a function of the resonance mass $m_{X}$. Some table entries are empty because resonance masses from 251 GeV to 5 TeV are considered, whereas some channels only use masses in a certain, more restricted, range.
Observed significance as a function of the resonance $m_{X}$. Some table entries are empty because resonance masses from 251 GeV to 5 TeV are considered, whereas some channels only use masses in a certain, more restricted, range.
Observed and expected upper limits at the 95% CL on the resonant Higgs boson pair production cross section as a function of the resonance mass $m_{X}$. Some table entries are empty because resonance masses from 251 GeV to 5 TeV are considered, whereas some channels only use masses in a certain, more restricted, range.
A summary of the constraints from searches performed by the ATLAS Collaboration for the electroweak production of charginos and neutralinos is presented. Results from eight separate ATLAS searches are considered, each using 140 fb$^{-1}$ of proton-proton data at a centre-of-mass energy of $\sqrt{s}$=13 TeV collected at the Large Hadron Collider during its second data-taking run. The results are interpreted in the context of the 19-parameter phenomenological minimal supersymmetric standard model, where R-parity conservation is assumed and the lightest supersymmetric particle is assumed to be the lightest neutralino. Constraints from previous electroweak, flavour and dark matter related measurements are also considered. The results are presented in terms of constraints on supersymmetric particle masses and are compared with limits from simplified models. Also shown is the impact of ATLAS searches on parameters such as the dark matter relic density and the spin-dependent and spin-independent scattering cross-sections targeted by direct dark matter detection experiments. The Higgs boson and Z boson `funnel regions', where a low-mass neutralino would not oversaturate the dark matter relic abundance, are almost completely excluded by the considered constraints. Example spectra for non-excluded supersymmetric models with light charginos and neutralinos are also presented.
SLHA files and exclusion information (in CSV format) are available to download for the pMSSM models in this paper. Please refer to <a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/SUSY-2020-15/inputs/ATLAS_EW_pMSSM_Run2.html">this web page</a> for download links along with a description of the contents.
SLHA files and exclusion information (in CSV format) are available to download for the pMSSM models in this paper. Please refer to <a href="https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/SUSY-2020-15/inputs/ATLAS_EW_pMSSM_Run2.html">this web page</a> for download links along with a description of the contents.
A search is presented for flavour-changing neutral-current interactions involving the top quark, the Higgs boson and an up-type quark ($q=u,c$) with the ATLAS detector at the Large Hadron Collider. The analysis considers leptonic decays of the top quark along with Higgs boson decays into two $W$ bosons, two $Z$ bosons or a $\tau^{+}\tau^{-}$ pair. It focuses on final states containing either two leptons (electrons or muons) of the same charge or three leptons. The considered processes are $t\bar{t}$ and $Ht$ production. For the $t\bar{t}$ production, one top quark decays via $t\to Hq$. The proton-proton collision data set analysed amounts to 140 fb$^{-1}$ at $\sqrt{s}=13$ TeV. No significant excess beyond Standard Model expectations is observed and upper limits are set on the $t\to Hq$ branching ratios at 95% confidence level, amounting to observed (expected) limits of $\mathcal{B}(t\to Hu)<2.8\,(3.0) \times 10^{-4}$ and $\mathcal{B}(t\to Hc)<3.3\,(3.8) \times 10^{-4}$. Combining this search with other searches for $tHq$ flavour-changing neutral-current interactions previously conducted by ATLAS, considering $H\to b\bar{b}$ and $H\to\gamma\gamma$ decays, as well as $H\to\tau^{+}\tau^{-}$ decays with one or two hadronically decaying $\tau$-leptons, yields observed (expected) upper limits on the branching ratios of $\mathcal{B}(t\to Hu)<2.6\,(1.8) \times 10^{-4}$ and $\mathcal{B}(t\to Hc)<3.4\,(2.3) \times 10^{-4}$.
Pre-fit background composition of the SR$2\ell$ Dec. The table shows the event yields as opposed to just the percentages of the relevant background processes.
Pre-fit background composition of the SR$2\ell$ Prod. The table shows the event yields as opposed to just the percentages of the relevant background processes.
Pre-fit background composition of the SR$3\ell$ Dec. The table shows the event yields as opposed to just the percentages of the relevant background processes.
Exclusive dimuon production in ultraperipheral collisions (UPC), resulting from photon-photon interactions in the strong electromagnetic fields of colliding high-energy lead nuclei, $\mathrm{PbPb}(\gamma\gamma) \rightarrow \mu^+\mu^- (\mathrm{Pb}^{(\star)}\mathrm{Pb}^{(\star)} )$, is studied using $\mathcal{L}_{\mathrm{int}} = 0.48$ nb$^{-1}$ of $\sqrt{s_\mathrm{NN}}=5.02$ TeV lead-lead collision data at the LHC with the ATLAS detector. Dimuon pairs are measured in the fiducial region $p_{\mathrm{T}\mu} > 4$ GeV, $|\eta_{\mu}| < 2.4$, invariant mass $m_{\mu\mu} > 10$ GeV, and $p_{\mathrm{T,\mu\mu}} < 2$ GeV. The primary background from single-dissociative processes is extracted from the data using a template fitting technique. Differential cross sections are presented as a function of $m_{\mu\mu}$, absolute pair rapidity ($|y_{\mu\mu}|$), scattering angle in the dimuon rest frame ($|\cos \vartheta^{\star}_{\mu\mu}|$) and the colliding photon energies. The total cross section of the UPC $\gamma \gamma \rightarrow \mu^{+}\mu^{-}$ process in the fiducial volume is measured to be $\sigma_{\mathrm{fid}}^{\mu\mu} = 34.1 \! \pm \! 0.3 \mathrm{(stat.)} \! \pm \! 0.7 \mathrm{(syst.)}$ $\mu\mathrm{b}$. Generally good agreement is found with calculations from STARlight, which incorporate the leading-order Breit-Wheeler process with no final-state effects, albeit differences between the measurements and theoretical expectations are observed. In particular, the measured cross sections at larger $|y_{\mu\mu}|$ are found to be about 10-20% larger in data than in the calculations, suggesting the presence of larger fluxes of photons in the initial state. Modification of the dimuon cross sections in the presence of forward and/or backward neutron production is also studied and is found to be associated with a harder incoming photon spectrum, consistent with expectations.
Differential UPC dimuon cross sections shown as a function of $|y_{\mu\mu}|$ in the interval $10 < |m_{\mu\mu}| < 20$ GeV.
Differential UPC dimuon cross sections shown as a function of $|y_{\mu\mu}|$ in the interval $20 < |m_{\mu\mu}| < 40$ GeV.
Differential UPC dimuon cross sections shown as a function of $|y_{\mu\mu}|$ in the interval $40 < |m_{\mu\mu}| < 80$ GeV.
The azimuthal variation of jet yields in heavy-ion collisions provides information about the path-length dependence of the energy loss experienced by partons passing through the hot, dense nuclear matter known as the quark-gluon plasma. This paper presents the azimuthal anisotropy coefficients $v_2$, $v_3$, and $v_4$ measured for jets in Pb+Pb collisions at $\sqrt{s_{NN}} =$ 5.02 TeV using the ATLAS detector at the LHC. The measurement uses data collected in 2015 and 2018, corresponding to an integrated luminosity of 2.2 nb$^{-1}$. The $v_n$ values are measured as a function of the transverse momentum of the jets between 71 GeV and 398 GeV and the event centrality. A nonzero value of $v_2$ is observed in all but the most central collisions. The value of $v_2$ is largest for jets with lower transverse momentum, with values up to 0.05 in mid-central collisions. A smaller, nonzero value of $v_3$ of approximately 0.01 is measured with no significant dependence on jet $p_T$ or centrality, suggesting that fluctuations in the initial state play a small but distinct role in jet energy loss. No significant deviation of $v_4$ from zero is observed in the measured kinematic region.
The JES for R = 0.2 jets in Pb+Pb collisions as a function of $p_T^{truth}$ for centrality selections of 0-5%, 5-10%, 10-20%, 20-40% and 40-60%.
The JER for R = 0.2 jets in Pb+Pb collisions as a function of $p_T^{truth}$ for centrality selections of 0-5%, 5-10%, 10-20%, 20-40% and 40-60%.
The JES for R = 0.2 jets in Pb+Pb collisions as a function of $2|\Psi_2-\phi^{reco}|$ for centrality selections of 0-5%, 5-10%, 10-20%, 20-40% and 40-60%.
Measurements of the suppression and correlations of dijets is performed using 3 $\mu$b$^{-1}$ of Xe+Xe data at $\sqrt{s_{\mathrm{NN}}} = 5.44$ TeV collected with the ATLAS detector at the LHC. Dijets with jets reconstructed using the $R=0.4$ anti-$k_t$ algorithm are measured differentially in jet $p_{\text{T}}$ over the range of 32 GeV to 398 GeV and the centrality of the collisions. Significant dijet momentum imbalance is found in the most central Xe+Xe collisions, which decreases in more peripheral collisions. Results from the measurement of per-pair normalized and absolutely normalized dijet $p_{\text{T}}$ balance are compared with previous Pb+Pb measurements at $\sqrt{s_{\mathrm{NN}}} =5.02$ TeV. The differences between the dijet suppression in Xe+Xe and Pb+Pb are further quantified by the ratio of pair nuclear-modification factors. The results are found to be consistent with those measured in Pb+Pb data when compared in classes of the same event activity and when taking into account the difference between the center-of-mass energies of the initial parton scattering process in Xe+Xe and Pb+Pb collisions. These results should provide input for a better understanding of the role of energy density, system size, path length, and fluctuations in the parton energy loss.
The centrality intervals in Xe+Xe collisions and their corresponding TAA with absolute uncertainties.
The centrality intervals in Xe+Xe and Pb+Pb collisions for matching SUM ET FCAL intervals and respective TAA values for Xe+Xe collisions.
The performance of the jet energy scale (JES) for jets with $|y| < 2.1$ evaluated as a function of pT_truth in different centrality bins. Simulated hard scatter events were overlaid onto events from a dedicated sample of minimum-bias Xe+Xe data.
Constraints on the Higgs boson self-coupling are set by combining double-Higgs boson analyses in the $b\bar{b}b\bar{b}$, $b\bar{b}\tau^+\tau^-$ and $b\bar{b} \gamma \gamma$ decay channels with single-Higgs boson analyses targeting the $\gamma \gamma$, $ZZ^*$, $WW^*$, $\tau^+ \tau^-$ and $b\bar{b}$ decay channels. The data used in these analyses were recorded by the ATLAS detector at the LHC in proton$-$proton collisions at $\sqrt{s}=13$ TeV and correspond to an integrated luminosity of 126$-$139 fb$^{-1}$. The combination of the double-Higgs analyses sets an upper limit of $\mu_{HH} < 2.4$ at 95% confidence level on the double-Higgs production cross-section normalised to its Standard Model prediction. Combining the single-Higgs and double-Higgs analyses, with the assumption that new physics affects only the Higgs boson self-coupling ($\lambda_{HHH}$), values outside the interval $-0.4< \kappa_{\lambda}=(\lambda_{HHH}/\lambda_{HHH}^{\textrm{SM}})< 6.3$ are excluded at 95% confidence level. The combined single-Higgs and double-Higgs analyses provide results with fewer assumptions, by adding in the fit more coupling modifiers introduced to account for the Higgs boson interactions with the other Standard Model particles. In this relaxed scenario, the constraint becomes $-1.4 < \kappa_{\lambda} < 6.1$ at 95% CL.
Observed and expected 95% CL upper limits on the signal strength for double-Higgs production from the bbbb, bb$\tau\tau$ and bb$\gamma\gamma$ decay channels, and their statistical combination. The value $m_H$ = 125.09 GeV is assumed when deriving the predicted SM cross-section. The expected limit and the corresponding error bands are derived assuming the absence of the HH process and with all nuisance parameters profiled to the observed data.
Observed and expected 95% CL exclusion limits on the production cross-sections of the combined ggF HH and VBF HH processes as a function of $\kappa_\lambda$, for the three double-Higgs search channels and their combination. The expected limits assume no HH production. The red line shows the theory prediction for the combined ggF HH and VBF HH cross-section as a function of $\kappa_\lambda$ where all parameters and couplings are set to their SM values except for $\kappa_\lambda$. The band surrounding the red cross-section lines indicate the theoretical uncertainty of the predicted cross-section.
Observed and expected 95% CL exclusion limits on the production cross-sections of the VBF HH process as a function of $\kappa_{2V}$, for the three double-Higgs search channels and their combination. The expected limits assume no VBF HH production. The red line shows the predicted VBF HH cross-section as a function of $\kappa_{2V}$. The bands surrounding the red cross-section lines indicate the theoretical uncertainty of the predicted cross-section. The uncertainty band is smaller than the width of the plotted line.
Studies of the correlations of the two highest transverse momentum (leading) jets in individual Pb+Pb collision events can provide information about the mechanism of jet quenching by the hot and dense matter created in such collisions. In Pb+Pb and pp collisions at $\sqrt{s_{_\text{NN}}}$ = 5.02 TeV, measurements of the leading dijet transverse momentum ($p_{\mathrm{T}}$) correlations are presented. Additionally, measurements in Pb+Pb collisions of the dijet pair nuclear modification factors projected along leading and subleading jet $p_{\mathrm{T}}$ are made. The measurements are performed using the ATLAS detector at the LHC with 260 pb$^{-1}$ of pp data collected in 2017 and 2.2 nb$^{-1}$ of Pb+Pb data collected in 2015 and 2018. An unfolding procedure is applied to the two-dimensional leading and subleading jet $p_{\mathrm{T}}$ distributions to account for experimental effects in the measurement of both jets. Results are provided for dijets with leading jet $p_{\mathrm{T}}$ greater than 100 GeV. Measurements of the dijet-yield-normalized $x_{\mathrm{J}}$ distributions in Pb+Pb collisions show an increased fraction of imbalanced jets compared to pp collisions; these measurements are in agreement with previous measurements of the same quantity at 2.76 TeV in the overlapping kinematic range. Measurements of the absolutely-normalized dijet rate in Pb+Pb and pp collisions are also presented, and show that balanced dijets are significantly more suppressed than imbalanced dijets in Pb+Pb collisions. It is observed in the measurements of the pair nuclear modification factors that the subleading jets are significantly suppressed relative to leading jets with $p_{\mathrm{T}}$ between 100 and 316 GeV for all centralities in Pb+Pb collisions.
absolutely normalized dijet cross sections from pp collisions
absolutely normalized dijet yields scaled by 1/<TAA> in 0-10% central PbPb collisions
absolutely normalized dijet yields scaled by 1/<TAA> in 10-20% central PbPb collisions
This paper describes a measurement of light-by-light scattering based on Pb+Pb collision data recorded by the ATLAS experiment during Run 2 of the LHC. The study uses $2.2$ nb$^{-1}$ of integrated luminosity collected in 2015 and 2018 at $\sqrt{s_\mathrm{NN}}=5.02$ TeV. Light-by-light scattering candidates are selected in events with two photons produced exclusively, each with transverse energy $E_{\mathrm{T}}^{\gamma} > 2.5$ GeV, pseudorapidity $|\eta_{\gamma}| < 2.37$, diphoton invariant mass $m_{\gamma\gamma} > 5$ GeV, and with small diphoton transverse momentum and diphoton acoplanarity. The integrated and differential fiducial cross sections are measured and compared with theoretical predictions. The diphoton invariant mass distribution is used to set limits on the production of axion-like particles. This result provides the most stringent limits to date on axion-like particle production for masses in the range 6-100 GeV. Cross sections above 2 to 70 nb are excluded at the 95% CL in that mass interval.
Measured differential fiducial cross sections of $\gamma\gamma \rightarrow \gamma\gamma$ production in Pb+Pb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV for diphoton invariant mass are shown as points with error bars giving the statistical uncertainty and grey bands indicating the size of the total uncertainty. The results are compared with the prediction from the SuperChic v3.0 MC generator (solid line) with bands denoting the theoretical uncertainty.
Measured normalised differential fiducial cross sections of $\gamma\gamma \rightarrow \gamma\gamma$ production in Pb+Pb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV for diphoton invariant mass are shown as points with error bars giving the statistical uncertainty and grey bands indicating the size of the total uncertainty. The results are compared with the prediction from the SuperChic v3.0 MC generator (solid line).
Measured differential fiducial cross sections of $\gamma\gamma \rightarrow \gamma\gamma$ production in Pb+Pb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV for diphoton $|cos(\theta*)|$ are shown as points with error bars giving the statistical uncertainty and grey bands indicating the size of the total uncertainty. The results are compared with the prediction from the SuperChic v3.0 MC generator (solid line) with bands denoting the theoretical uncertainty.
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